How Caustic Scrubber Chemistry Affects Mist Eliminator Fouling and Material Selection

A caustic wet scrubber may appear to be a relatively simple mist eliminator application.

Gas passes through an alkaline scrubbing liquid, droplets are carried upward, and a demister removes the entrainment before the gas leaves the vessel.

In practice, caustic scrubber service can become difficult because the chemistry does not stop at the spray nozzle or packed bed.

The liquid reaching the mist eliminator may contain:

• dissolved salts;
• reaction products;
• suspended solids;
• concentrated alkaline solution.

As captured droplets drain, evaporate, or react with the gas, deposits can form directly inside the separator.

For this reason, caustic scrubber demister design should consider not only corrosion resistance but also reaction chemistry, salt formation, crystallization, and washability.

Why Caustic Scrubbers Can Generate Salts

Caustic scrubbers commonly use alkaline solutions to absorb acidic gas components.

Depending on the process, reactions can generate dissolved salts.

The scrubber circulation liquid may remain fully liquid under normal bulk conditions.

However, the situation inside the mist eliminator can be different.

Captured droplets form thin films on:

• wires;
• vane surfaces;
• support members.

If water evaporates from these films, dissolved species become more concentrated.

Once the local concentration exceeds solubility, solid deposits can form.

This means a scrubber with no visible solids in the sump can still develop crystalline fouling inside the demister.

Carbonate Formation Can Matter

Caustic solutions exposed to carbon dioxide can form carbonate species.

In some operating conditions, these reaction products can contribute to scaling or solids formation.

The exact chemistry depends on:

• caustic concentration;
• gas composition;
• temperature;
• pH.

The important engineering point is that the mist eliminator sees the final reacted liquid—not simply fresh NaOH solution.

A material or mesh specification based only on fresh chemical concentration may therefore miss the actual fouling environment.

Fine Wire Mesh Can Concentrate the Problem

Wire mesh provides a large collecting surface.

This is excellent for droplet interception.

It also provides many locations where:

• alkaline liquid can remain;
• water can evaporate;
• crystals can nucleate.

Once deposits begin forming, they reduce the mesh openings.

Pressure drop rises.

Drainage becomes poorer.

The retained liquid becomes more concentrated.

More deposits form.

This creates a self-reinforcing fouling cycle.

High pH Does Not Mean Fouling Cannot Occur

Operators sometimes focus almost entirely on corrosion because alkaline liquids are associated with material compatibility.

But a chemically resistant separator can still become hydraulically unusable.

PP or another compatible polymer may remain chemically intact while the mesh passages become filled with salt deposits.

Chemical survival and hydraulic reliability are separate design questions.

The correct separator must achieve both.

Liquid Concentration Changes Across the Scrubber

The liquid composition near the spray nozzles may not be the same as the liquid reaching the demister.

As gas absorption proceeds:

• contaminants react;
• pH changes;
• salt concentration changes.

If the scrubber also experiences evaporation, the liquid can become more concentrated.

Therefore, the demister should ideally be evaluated using the expected composition in the upper part of the vessel—not merely the makeup-liquid concentration.

Material Selection Still Matters

Common separator materials may include:

• PP;
• PVDF;
• SS304;
• SS316L;
• other alloys or polymers.

The correct choice depends on actual:

• chemical composition;
• temperature;
• concentration;
• contaminants.

A generic statement such as “NaOH service” is not enough.

Concentrated alkaline service at elevated temperature can differ greatly from dilute ambient scrubber operation.

The support grid, frame, and fasteners require the same compatibility review.

Salt Formation Can Favor a More Open Separator

If fouling risk is significant, a very dense mesh may provide excellent clean-condition droplet removal but short operating life.

A more open geometry can offer:

• larger gas passages;
• better drainage;
• easier washing.

Depending on droplet size and liquid load, possible options may include:

• open wire mesh;
• vane separation;
• staged vane plus mesh.

The objective is not simply to maximize initial efficiency.

It is to maintain acceptable performance between maintenance shutdowns.

Wash-System Design Is Important

Caustic scrubber deposits may be water-soluble, but washability depends on chemistry.

A successful washing system needs:

• adequate coverage;
• suitable water quality;
• enough wash volume;
• effective drainage.

If the wash water merely wets the outer surface while internal deposits remain, pressure drop may not recover.

Likewise, contaminated recycled wash water can introduce additional dissolved solids.

Cleaning performance should be evaluated by pressure-drop recovery, not only by visual appearance.

High Liquid Circulation Can Increase Demister Burden

Increasing caustic circulation may improve absorption performance.

But it can also increase:

• spray entrainment;
• packed-bed liquid holdup;
• demister liquid load.

This creates an interaction between gas-removal efficiency and mist-separation performance.

A process change intended to improve acid-gas absorption can therefore create new outlet liquid carryover if the mist eliminator no longer has sufficient drainage capacity.

Foaming Can Make the Situation Worse

Some contaminated alkaline systems can foam.

Foam collapse can create fine droplets.

Foam can also carry salts and solids toward the mist eliminator.

The separator then faces:

• higher liquid loading;
• finer droplets;
• greater fouling.

If carryover appears suddenly after a chemistry change, foam behavior should be reviewed together with demister condition.

Differential Pressure Provides an Early Warning

As deposits reduce open area, differential pressure typically rises.

The most useful trend compares DP under similar:

• gas flow;
• liquid circulation;
• temperature.

A gradual increase at the same operating condition can indicate developing fouling.

Cleaning before severe plugging generally provides better recovery than waiting until production is limited.

Inspect Deposit Chemistry

During shutdown, collect and identify deposits where practical.

Questions include:

• Are they crystalline?
• Are they soluble in water?
• Are they mixed with solids?
• Are they concentrated near support members?

Deposit composition can reveal whether the root problem is:

• reaction chemistry;
• water quality;
• upstream solids;
• poor drainage.

Repeated cleaning without understanding the deposit source may only shorten the interval to the next shutdown.

What Should Be Included in an RFQ?

Useful information includes:

• caustic concentration;
• absorbed gas species;
• expected reaction products;
• temperature;
• gas flow;
• liquid circulation;
• solids;
• fouling history;
• wash availability;
• tower diameter.

If detailed salt chemistry is unknown, historical plant behavior can still provide valuable guidance.

Final Engineering Perspective

A caustic scrubber mist eliminator operates inside a reacting chemical system.

The liquid reaching the separator may contain much more than NaOH.

Reaction products, salts, evaporation, solids, and foaming can all change separator behavior.

The best design therefore balances chemical compatibility, droplet capture, drainage, fouling tolerance, and cleanability.